Method of electrically induced deposition of paint on conductors



May 13, 1969 BREWER ET AL 3,444,066

METHOD OF ELECTRICALLY INDUCED DEPOSiTIQN 0F PAINT ON CONDUCTORS Filed De c. v, 1966 Sheet of s F/G. z

GEO/P65 5f. BREW/E78 6/4 545974. BUR V5705 ARA/04F E M0694 GO/PDO/V 6.5720555/Q6 INVENTORS May 13, 1969 G. E. F. BREWER ET AL 3,444,066

METHOD OF ELECTRICALLY INDUCED DEPOSITION 0F PAINT ON CONDUCTORS Filed Dec. 7, 1966 Sheet 2 of s g kg Q "I L, [hipl l I' T fi lrT IIIYIIT/IITIIIII\ }IIIfI/IIITI I I I l ii J I" A} R t\ I I "K I I l: I

l I I I I I I I "g a l r IF FI I I m I" Q l I I I I 4 II I *3 I II t 9 I .RTVF- .1 Ill I I 4 F I PI J I mI E I innlfltullu g I I I I L twir :3 I I: F; Q

\1 Ii 9 k 660/4 65 5 E BREWER M} k or 6/1 35w- 4. sum/.5705 k 5mg A/P/VZ/LF/ T/WO/IA/Q Q Q E v, 60200 6. 574 053596 Q 8; 6 8 INVENTORJ a z 9% /P% I BY May 13, 1969 s. E. F. BREWER ET AL 3,444,066

METHOD OF ELECTRICALLY INDUCED DEPOSITION 0F PAINT 0N CONDUCTORS Sheet Filed Dec. 7, 1966 ATTORNEYS 3,444,066 METHOD OF ELECTRICALLY INDUCED DEPOSI- TION OF PAINT N CONDUCTORS George E. F. Brewer, Novi, Gilbert L. Burnside, Oak Park, Arnulf F. Mohar, Dearborn Heights, and Gordon G. Strosberg, Oak Park, Mich., assignors to Ford Motor Company, Dearborn, MlClL, a corporation of Delaware Filed Dec. 7, 1966, Ser. No. 599,959

Int. Cl. B01k 5/02 U.S. Cl. 204-181 Claims This invention relates to the art of electrocoating electrically conductive objects by electrically induced deposition from an aqueous bath of a coating material cornprising an organic resin having functional groups in its molecular structure which are ionizable in aqueous medium. In such a process, the resin or film former and any other coating material components employed, e.g., p gment, are dispersed in an aqueous bath with a water-dissociable dispersal assistant which may also be termed a solubilizer. The object to be coated is immersed in the bath and a difference of electrical potential is provided between such object and another electrode in contact with the bath sufiicient to effect electrodeposition of a water resistant coating of said coating material upon the object.

The instant process is useful with either an anodic or a cathodic deposition process. Resins suitable for anodic electropainting are exemplified by polycarboxylic acid resins which are dispersed in the aqueous coating bath with the aid of a water soluble amino compound, such term herein including water soluble amines and amrnonia. A resin suitable for cathodic deposition has in its molecular structure ionizable groups which upon ionization leave ionically positive sites. Examples of such groups are amine and substituted amine groups such as quaternary ammonium groups. Dispersion of such resins is effected by the addition of water-ionizable acidic dispersal assistants, e.g., water soluble carboxylic acids and suitably buffered forms of certain inorganic acids. Buffered phosphoric acid is exemplary of the latter. To date anodic deposition has gained primary acceptance in this field and is exemplified by U.S. Patent 3,230,162 to A. E. Gilchrist, which patent is herein incorporated by reference. This embodiment of the electrocoating process is used herein for the detailed description of the invention, it being understood that the polarity of the electrodes shown in the drawings are reversible for cathodic deposition embodiments.

In the process of electrocoating, the film-forming resin depletes from the bath as a succession of conductive objects are immersed therein and painted. Ions of the dispersal assistant or solubilizer are attracted to the electrode of opposite polarity and are redistributed through the bath. A fraction of the solubilizer is lost by evaporation, drag-out with the objects being coated, etc. This loss commonly amounts to approximately of the solubilizer introduced into the bath with approximately 75% left to accumulate since the replenishment feed of coating material also contains a quantity of solubilizer. This accumulation of solubilizer causes the pH of the bath to change in proportion to the accumulation, an increase in current consumption, a decrease in throwing power, an increase in gas evolution at the electrodes, and the coating deposited after such accumulation increases in roughness, decreases in resistance to electrically induced film rupture, and decreases in ability to provide corrosion protection.

It has been suggested to remove the excess solubilizer by dialysis, electrodialysis, and ion exchange. This ordinarily results in loss of the solubilizer thus removed. It is also known to allow the solubilizer to accumulate to a high, but tolerable, level, and then use the solubilizer enriched bath for the dispersion of fresh coating material.

3,444,%6 Patented May 13, 1969 The high solids replenishment feed although ordinarily containing some amine still requires additional amine for introduction into the coating bath. This method allows dispersion of the replenishment feed quickly by passing only a small portion of the bath without modification, i.e. whole bath, through a homogenizer with the replenishment feed but results in operating with a solubilizer concentration markedly above optimum concentrations for coating with resultant quality loss.

It is one object of this invention to provide an improved method for replenishment of coating material in a continuous electrocoating process without maintaining an excessive concentration of solubilizer in the coating bath or separating from the coating materials of said bath significant amounts of solubilizer.

It is another object of this invention to provide method and means to simultaneously maintain optimum pH in an electrocoating bath, effect dispersal of replenishment feed and minimize solubilizer requirements.

It is another object of this invention to provide method and means for withdrawing a solubilizer enriched portion of an electrocoating bath, dispersing replenishment feed within said portion, and returning the resultant dispersion to the coating bath.

With the foregoing and other objects in view, as will hereinafter become apparent, this invention comprises the methods, combinations, construction and arrangement of parts hereinafter set forth, disclosed, claimed and illustrated in the accompanying drawings, wherein:

FIGURE 1 is a schematic plan view of one embodiment of electrocoating apparatus which can be used to carry out the method of this invention;

FIGURE 2 is a sectional view of the apparatus shown in FIGURE 1 taken along line 22 showing additional equipment positioned outside the view of FIGURE 1;

FIGURE 3 is an exploded view of one of the cathode assemblies of the coating tank shown in FIGURES 1 and 2.

Referring now to FIGURES 1-3, there is shown an electrocoating tank 10 which comprises a metal tank 11 for structural support and a nonconductive liner 13, e.g., a suitable plastic material, a polymerized organic coating such as a cured paint film, etc., which defines the coating compartment 15. Positioned alOng the walls of tank 10 are a plurality of cathode assemblies 17 shown in detail in FIGURE 3. Each of the cathode assemblies 17 comprises a cathode support 17-1 that is affixed by conventional means, e.g., bolts, rivets, clamps, etc., to the inner wall of tank It the latter forming one side of the resulting cathode compartments 17-2. A wire screen cathode 17-3, diffusion barrier 17-4 and frame 17-5 define the side of the cathode compartment facing the center of the coating compartment 15. Frame 17-5 is here employed to secure cathode 17-3 and filter 17-4 to cathode support 17-1. In one embodiment, the effective cathode surface area is much smaller than the anode surface area within the coating bath at any given time, e.g., advantageously below about 25%, preferably below about 10% Each of the cathode assembly units 17 have an outlet conduit 17-6. Catholyte is removed from each of the compartments 17 via conduits 17-6 which empty into a common catholyte collection unit 19. The catholyte is removed from catholyte collection unit 19 via conduit 21 and pumped via pump 23 and conduit 25 to paint dispersal unit 27. Replenishment feed is introduced into paint dispersal unit 27, as, for instance, via conduit 29, mixed with the catholyte introduced via conduit 25 and returned via conduit 31 to the coating bath 33 in compartment 15. It will be understood that the positioning of conduit 31 is merely exemplary and that it is within the scope of this invention to return the replenishment feed stream to any level of bath 33.

Referring now to the coating bath 33, there is shown immersed therein an article 35. Article 35 is any electrically conductive workpiece to be painted and in this embodiment serves as the anode of the coating cell. Article 35 is shown suspended from a hanger 37 which in turn is carried by an overhead conveyor 39. Article 35 and hanger 37 are electrically insulated from conveyor 39 by insulator 41. Conveyor 39 will normally be at ground potential. Article 35 is in electrical connection with a positive terminal of D.C. power source 49 via conductor brush 43, bus bar 45 and conductor 47. The electrical circuit for coating is completed by conductor 51 which places each of the cathodes 15-3 in electrical connection with a negative terminal of power source 49. A difference of electrical potential of above about 50 volts, more commonly in the range of about 100 to about 250 volts or higher, is provided between anode and cathode during coating. The upper limit is ordinarily determined by the electrical rupture resistance characteristics of a given paint.

Diffusion barrier 17-4 difl'ers from a dialysis membrane in that it is permeable to significant amounts of paint solids. The pore size is suiflciently restricted to retard movement of the bath to a degree that slows redispersal of solubilizer entering the cathode assembly, provides a concentration of solubilizer within the cathode assembly or assemblies significantly higher than that of the total bath, and provides therein a solids concentration significantly below that of the total bath. While the optimum porosity will vary somewhat with the constituents of a given paint bath, the pumping rate from the cathode compartment, and the electrical energy flows between anode and cathodes (coulombs), a /8 thick viscose sheet with 10 micron pore size has proven quite satisfactory with electrocating paints now available. Other materials suitable for use as the dispersion barrier includes nylon, Teflon, glass, etc.

The method of this invention is more readily understood from an operational material balance of an exemplary flow pattern in accordance with this method wherein the paint comprises a polycarboxylic acid resin binder and coating is effected by anodic deposition. Assume, for instance, a 20,000 gram aqueous coating bath containing about 8 wt. percent paint solids or more properly termed nonvolatiles, hereinafter referred to as NV and about 82 milliequivalents amine, hereinafter termed MEQA, per 100 grams NV. About 2,000 grams catholyte per unit time is removed from the cathode chambers containing about wt. percent NV, i.e., about 100 grams NV and about 110 MEQA per 100 grams NV. To this removed catholyte is fed about 101 grams of 85.5 grams NV and about 17.1 MEQA. The catholyte plus feed in the amount Sufficient total amine is employed in the system to maintain the amine concentration of the bath which contacts the anode at the desired operational level and to provide a concentration within the catholyte that is sufficient to solubilize, i.e., place in condition for introduction into the coating bath, a quantity of replenishment feed sufiicient to maintain the system in balance. In this method, the amine concentration of the return flow of catholyte plus feed does not exceed the equilibrium concentration of amine in the working bath plus operational loss of amine for the paint-out of an amount of NV equal to that of the feed. In a preferred embodiment, this ilow is returned to the bath with a lesser concentration of amine than the equilibrium concentration of the coating bath. Advantageously, the catholyte removed contains NV in a concentration equal to about 40 wt. percent to about wt. percent of the NV concentration of the bath in contact with the anode at equilibrium and the return feed stream advantageously contains NV in a concentration equal to about 15 wt. percent to about 35 wt. percent of the NV concentration of the catholyte. The concentration of amine, MEQA, in the catholyte, however, is advantageously in excess of about 20% and preferably in excess of about 30% higher than that of the bath in contact with the anode at equilibrium. Concentrations of amine in catholyte in the range of about 30% to about 40% higher than that of the bath have proven quite satisfactory. In some systems, moderate increases in this concentration, e.g., up to an additional 3 to 7%, may find advantageous use.

The advantages of the method will be more fully understood from the following illustrative examples:

Example 1 The method of electrocoating and feed replenishment as hereinbefore described was tested with fresh coating material, a commercially available electrodepositable automobile primer of which the binder resin is a polycarboxylic acid resin. To a nonconductive coating tank a 20 liter bath containing about 8 wt. percent NV was added. The cathode compartment held about 1 liter of fluid when placed in fluid communication with the coating bath. The porous or intake side of the cathode compartment measured 390 cm. and the filter screen employed was a A" thick viscose sheet with 10 micron pore size. The rate of catholyte removal was 200 milliliters per minute or approximately 1% of the bath. This was pumped to a feed mixing tank of about 3 liters capacity. Sheet steel anode, workpiece to be coated, was fed to the coating bath at a rate of about 2 square feet per minute. Further operational details and results are set forth in the following table:

TABLE 1.-ELECTROGOATING FROM INIT'IATION WITH lNggflBATH-REPLENISHMENT FEED TO WITHDRAWN CATHO- Bath Catholyt-e Feed Coating Conditions Film Anode NV, NV, MEQA/ NV, MEQA/100 NV, Depth, Appear- C oated, it. gins. percent pH gins. NV percent pH gms. NV gms. MEQA Volts Amps mils once 1 1, 640 8.65 9.7 86.5 9.75 9.7 86.5 0 0 190 3.0 0.6-0 .7 Smooth. 1, 360 1,350 7.10 9 .1 82 .6 5 .50 9 .8 111 .0 2, 693 500 190 3 .0 0 .6-0 .7 D0.

of about 2101 grams is returned to the bath containing about 8.8 wt. percent NV and about 69 MEQA per 100 grams NV or a total of about 185.5 grams NV and about 127.1 MEQA. It will be understood that the foregoing illustrates continuous operation in which workpiece surface area is processed, i.e., painted, at a rate such that the paint-out or operational loss of NV is equivalent to the feed addition of NV.

In contrast a run, is made by the conventional process of allowing amine buildup in the bath allowing replenishment feed to be made to a portion of the whole bath. In this run the paint employed was the same automobile primer aforereferred to in the above illustration of electrocoating in accordance with the method of this invention. The conditions employed and the results obtained are set forth in the following table:

TABLE 2.ELECTROCOATING RUN FROM INITIATION WITH NEW BATH-REPLENISHMENT FEED DIRECTLY TO BATH Bath Feed Coating Conditions Film Anode NV NV, MEQA/100 Depth, Coated, ft. gins. percent pH gms. NV NV, gms. MEQA Volts Amps mils Appearance 1 1, 490 7.9 8.9 90.4 0 0 200 5 .0 0.6-0.7 Smooth. 1, 400 1, 480 7 .8 9 .3 95 .0 2, 390 600 6 .0 0 .4-1 .0 Rough-with ruptures.

The method of the invention was demonstrated as in Example 1, same conditions except where otherwise indicated, using as the starting bath a portion of a large scale production bath taken from a 46,000 gallon coating tank. The coating material of the starting bath was the same as in the previous example except that this bath had experienced approximately 23 turnovers over a period of 1.5 years. The large bath had been initiated at about 8.7% NV. With time this material had been sweetened with fresh paint solids to about 9.75% NV to maintain as nearly as possible initial quality of coating. A coating obtained from material taken directly from the large bath was cured by baking. This bath was then operated in accordance with the method of this invention until the film appearance became relatively smooth at the original concentration of 8.7% NV. The conditions of operation for this run and the results are set forth in the following table:

dispersed within said bath with an ionized dispersal assistant, said method comprising immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as a first electrode of an electrical circuit comprising said bath, said first electrode, and a second electrode in contact with said bath and spaced apart from said object, providing a difference of electrical potential between said first electrode and said second electrode sufficient to cause a direct current of electrical energy through said bath and between said first electrode and said second electrode having direction and sufliciency to elfect electrodeposition of a coating of said paint upon said object from said bath, the improvement which comprises interposing a diffusion barrier admitting of liquid flow therethrough between said first electrode and said second electrode forming a bath-withdrawal zone, withdrawing from said zone aqueous electrolyte of reduced paint concentration and increased concentration TABLE 3.ELECTROCOATING RUN FROM INITIATION WITH OPERATIONALLY AGED BATH-REPLENISHMENT FEED TO WITHDRAWN CATHOLYTE Bath Catholyte Feed Coating Conditions Film Anode NV, MEQA/IOO NV, MEQA/IOO NV, Depth, Appear- Coated, it? gms. percent pH gms. NV percent pH gms. NV gms. MEQA Volts Amps mils ance 1 1, 840 9 .75 9 .1 97 .0 9 .75 9 .1 97 .0 0 0 175 4 .2 0 .4-0 .6 Rough. 1, 360 1, 645 8 .70 8 .9 84 .6 7 .0 9 .6 106 .5 2, 145 280 210 3 .0 0.6-0 .7 Smooth.

In this application, painting by electrodeposition is meant to include the deposition of finely ground pigment and/or filler in the ionizable resin herein referred to as the binder, the deposition of binder without pigment and/or filler or having very little of same, but which can be tinted if desired, and the deposition of other water reducible surface coating compositions containing the binder which might be considered to be broadly analogous to enamel, varnish, or lacquer bases, and the coating material for such deposition is termed a paint. Thus, the binder, which is converted to a water-resistant film by the electrodeposition and ultimately converted to a durable film resistant to conventional service conditions by final curing, can be all or virtually all that is to be deposited to form the film, or it can be a vehicle for pigmentary and/or mineral filler material or even other resins on which it exerts the desired action for depositing the film. Suitable resins include but are not limited to those specifically listed in US. Patent 3,230,162 to A. E. Gilchrist. The preferred resins for anodic deposition have an acid number between about 30 and about 300 and an electrical equivalent weight between about 1,000 and about 20,000. The term electrical equivalent weight is employed herein to mean that amount of resin or resin mixture that will deposit per Faraday of electrical energy input. The conditions, procedures, and calculations which can be employed to determine electrical equivalent weight are set forth in detail in the aforementioned US Patent 3,230,162.

The term milliequivalent as employed herein in the examples of anodic deposition refers to that amount of base which when admixed with 36.47 milligrams of HCl in 1000 milliliters water containing 100 grams NV of anodically depositable paint will produce a bath pH of 3.5.

Weight percent nonvolatiles is determined herein by evaporating the water from a weighed quantity of coating bath, baking the remainder at 350 F. for minutes, weighing the residue, and calculating the weight percentage of said residue in relation to the original weight of the sample.

The foregoing examples are solely for purposes of illustration and should not be considered as limitations upon the true scope of the invention as set forth in the appended claims.

We claim:

1. In a method for coating an electrically conductive object in an aqueous bath with a paint comprising an organic resin having ionized sites thereon and intimately of dispersal assistant relative to the corresponding concentrations of said bath where said bath is in contact with said first electrode, admixing said electrolyte with replenishment paint for said bath and returning the resultant replenishment feed to said bath.

2. The method of claim 1 wherein said electrically conductive objects are continuously introduced into said bath and said replenishment feed is introduced into said bath at a rate sufiicient to maintain the concentration of paint and dispersal assistant in said bath at substantially constant levels.

3. In a method for coating an electrically conductive object in an aqueous bath with a paint comprising a polycarboxylic acid resin having ionized sites thereon and intimately dispersed within said bath with ionized amine, said method comprising immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as an anode of an electrical circuit comprising said bath, said anode and a cathode in contact with said bath and spaced apart from said anode, providing a dilference of electrical potential between said anode and said cathode sufficient to cause a direct current of electrical energy through said bath and between said anode and said cathode having direction and sufiiciency to effect electrodeposition of a coating of said paint upon said anode from said bath, the improvement which comprises interposing a diifusion barrier admitting of liquid flow therethrough between said anode and said cathode forming a cathode zone otherwise separated from the bath in an anode zone wherein said anode is in contact with said bath, withdrawing from said cathode zone aqueous catholyte of reduced paint concentration and increased concentration of said amine relative to the corresponding concentrations of said paint and said amine in said bath in said anode zone, admixing said catholyte with replenishment paint for said bath and returning the resultant replenishment feed to said bath in said anode zone.

4. The method of claim 3 wherein said catholyte contains said paint in a concentration equal to about 40 wt. percent to about 60 wt. percent of the concentration of said paint in said bath in said anode zone and said replenishment feed contains a paint concentration that is about 15 wt. percent to about 35 wt. percent above that of said catholyte.

S. The method of claim 3 wherein the concentration of said amine in said catholyte is in excess of about 20% higher than the concentration of said amine in said bath in said anode zone.

6. The method of claim 3 wherein the concentration of said amine in said catholyte is about 30% to about 40% higher than the concentration of said amine in said bath in said anode zone.

7. The method of claim 3 wherein said replenishment feed contains a lower concentration of said amine than said bath in said anode zone.

8. The method of claim 3 wherein said cathode has an effective surface area which is less than about 25% of the surface area of said anode.

. 9. The method of claim 3 wherein said cathode has an effective surface area which is not greater than about 10% of the surface area of said anode.

10. The method of claim 1 wherein said cathode comprises a plurality of conductors each positioned within a separate cathode zone and catholyte is withdrawn from said bath in a plurality of streams which are conveyed to a mixing zone wherein said replenishment paint is admixed therewith.

References Cited HOWARD S. WILLIAMS, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner.

US. Cl. X.R. 204-301 

1. IN A METHOD FOR COATING AN ELECTRICALLY CONDUCTIVE OBJECT IN AN AQUEOUS BATH WITH A PAINT COMPRISING AN ORGANIC RESIN HAVING IONIZED SITES THEREON AND INTIMATELY DISPERSED WITHIN SAID BATH WITH AN IONIZED DISPERSAL ASSISTANT, SAID METHOD COMPRISING IMMERISNG SAID OBJECT WITHIN SAID BATH, UTILIZING SAID BATH AS THE AQUEOUS ELECTROLYTE AND SAID OBJECT AS A FIRST ELECTRODE OF AN ELECTRICAL CIRCUIT COMPRISING SAID BATH, SAID FIRST ELECTRODE, AND A SECOND ELECTRODE IN CONTACT WITH SAID BATH AND SPACED APART FROM SAID OBJECT, PROVIDING A DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE SUFFICIENT TO CAUSE A DIRECT CURRENT OF ELECTRICAL ENERGY THROUGH SAID BATH AND BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE HAVING DIRECTION AND SUFFICIENCY TO EFFECT ELECTRODEPOSITION OF A COATING OF SAID PAINT UPON SAID OBJECT FROM SAID BATH, THE IMPROVEMENT WHICH COMPRISES INTERPOSING A DIFFUSION BARRIER ADMITTING OF LIQUID FLOW THERETHROUGH BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE FORMING A BATH-WITHDRAWAL ZONE, WITHDRAWING FROM SAID ZONE AQUEOUS ELECTROLYTE OF REDUCED PAINT CONCENTRATION AND INCREASED CONCENTRATION OF DISPERSAL ASSISTANT RELATIVE TO THE CORRESPONDING CONCENTRATIONS OF SAID BATH WHERE SAID BATH IS IN CONTACT WITH SAID FIRST ELECTRODE, ADMIXING SAID ELECTROLYTE WITH REPLENISHMENT PAINT FOR SAID BATH AND RETURNING THE RESULTANT REPLENISHMENT FEED TO SAID BATH. 